It takes a certain amount of oxygen(air) to completely burn a certain amount of fuel. This is the optimal air/fuel ratio and I think its either 14-1 or 17-1. Either way, most cars run it a little lean in order to decrease harmful emmissions and to preserve the catalytic converter. However, if you keep it at the optimal ratio, you'll get the best performance. It really shouldn't matter weather or not its a boosted vehicle, the ratio should be the same.

I don't see how much of a real performance upgrade you would get by running dual exhaust off of a single pipe, but to answer the question asked, someone else said to just connect the o2 sensor to the single pipe and they are right. That should fix the problem.